Infrared detector having at least one switch for modulation and/or bypass

ABSTRACT

An infrared (IR) detector including a plurality of thermal sensing elements for generating an image of an object is provided. The IR detector comprises a first thermal sensing element and includes a thermopile and a first switch. The thermopile is configured to receive at least a portion of a thermal output from the object and to provide a modulated electrical output indicative of at least a portion of the received thermal output. The first switch is operatively coupled to the thermopile and is configured to provide a bypass in the event the thermopile is damaged such that remaining thermal sensing elements of the plurality of thermal sensing elements are capable of providing an electrical output therefrom.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of U.S. application Ser. No.13/270,852 filed Oct. 11, 2011, which, in turn, claims the benefit ofU.S. provisional Application No. 61/392,005 filed on Oct. 11, 2010 andis a continuation in part of co-pending U.S. application Ser. No.13/263,825, filed on Oct. 10, 2011, which is the U.S. National Phase ofPCT application No. PCT/US2010/028293, which in turn, claims the benefitof U.S. provisional application Ser. No. 61/168,601 filed on Apr. 12,2009; the disclosures of which are hereby incorporated by reference intheir entirety.

TECHNICAL FIELD

Embodiments of the present invention generally relate to, among otherthings, an infrared (IR) detector.

BACKGROUND

An IR detector is generally defined as a photodetector that responds toIR radiation. One type of an infrared detector is a thermal baseddetector. A thermal based detector may be implemented within a camera togenerate an image of an object formed on the thermal propertiesgenerally associated with such an object. Thermal based detectors areknown to include bolometers, microbolometers, pyroelectric, andthermopiles.

A microbolometer changes its electrical resistance based on an amount ofradiant energy that is received from an object. Thermopiles include anumber of thermocouples that convert thermal energy from the object intoelectrical energy. Such devices have been incorporated into cameras inone form or another for thermal imaging purposes.

SUMMARY

An infrared (IR) detector including a plurality of thermal sensingelements for generating an image of an object is provided. The IRdetector comprises a first thermal sensing element and includes athermopile and a first switch. The thermopile is configured to receiveat least a portion of a thermal output from the object and to provide amodulated electrical output indicative of at least a portion of thereceived thermal output. The first switch is operatively coupled to thethermopile and is configured to provide a bypass in the event thethermopile is damaged such that remaining thermal sensing elements ofthe plurality of thermal sensing elements are capable of providing anelectrical output therefrom.

BRIEF DESCRIPTION OF THE DRAWINGS

The embodiments of the present invention are pointed out withparticularity in the appended claims. However, other features of thevarious embodiments will become more apparent and will be bestunderstood by referring to the following detailed description inconjunction with the accompany drawings in which:

FIG. 1 depicts a conventional microbolometer based detector;

FIG. 2 depicts a conventional thermopile based detector;

FIG. 3 depicts an IR detector in accordance to one embodiment of thepresent invention;

FIG. 4 depicts a function generator implemented within the IR detectorof FIG. 3 in accordance to one embodiment of the present invention;

FIG. 5 depicts a thermopile array implemented within the IR detector ofFIG. 3 in accordance to one embodiment of the present invention;

FIG. 6 depicts another thermopile array implemented within the IRdetector of FIG. 3 in accordance to one embodiment of the presentinvention;

FIG. 7 depicts another IR detector in accordance to one embodiment ofthe present invention;

FIG. 8 depicts a thermopile array implemented within the IR detector ofFIG. 7 in accordance to one embodiment of the present invention;

FIG. 9 depicts a thermopile and switching arrangement for a voltagesumming configuration in accordance to one embodiment of the presentinvention;

FIG. 10 depicts another thermopile and switching arrangement for thevoltage summing configuration in accordance to one embodiment of thepresent invention;

FIG. 11 depicts another thermopile and switching arrangement for thevoltage summing configuration in accordance to one embodiment of thepresent invention;

FIG. 12 depicts a thermopile and switching arrangement for a currentsumming configuration in accordance to one embodiment of the presentinvention;

FIG. 13 depicts another thermopile and switching arrangement for acurrent summing configuration in accordance to one embodiment of thepresent invention; and

FIG. 14 depicts another thermopile and switching arrangement for thecurrent summing configuration in accordance to one embodiment of thepresent invention.

DETAILED DESCRIPTION

Detailed embodiments of the present invention are disclosed herein.However, it is to be understood that the disclosed embodiments aremerely exemplary of the invention that may be embodied in various andalternative forms. The figures are not necessarily to scale; somefeatures may be exaggerated or minimized to show details of particularcomponents. Therefore, specific structural and functional detailsdisclosed herein are not to be interpreted as limiting, but merely as arepresentative basis for the claims and/or as a representative basis forteaching one skilled in the art to variously employ the one or moreembodiments of the present invention.

Various embodiments of the present invention generally provide for, butnot limited to, an IR detector that includes a thermal sensing devicebased array. The array includes a plurality of thermal sensing elementsthat each include a thermopile (or other suitable thermal sensingdevice) distributed into M columns and N rows (e.g., M×N thermopilearray). A function generator (or other suitable device that is situatedto generate an oscillating signal at a corresponding frequency) maydrive each column (or row) of thermal sensing elements (to modulate anoutput of each thermopile) within the array with oscillating signals ata different frequency from one another such that an electrical output isprovided for each column (or row). The modulated electrical output fromeach thermopile in the column (or row) may be provided on a singlemodulated electrical output and is amplified by an amplifier (or othersuitable device) for the given column (or row). A demodulation circuitmay receive each single modulated electrical output after amplificationfor each column (or row) and demodulate the amplified output (e.g.,remove constant value from oscillating signal(s) for each column (orrow)) to generate a constant electrical value. The constant electricalvalue may be indicative of a portion of the entire the detected image.The entire detected image can be reconstructed by assembling all of theconstant electrical values that are read from each column (or row)within the array. It is recognized that it may not be necessary to driveall thermal sensing elements in every row and column in the array andthat selected clusters of the thermal sensing elements in acorresponding column (or row) may be driven with oscillating signals ata different frequency from one another. This condition may reduce costof the IR detector in the event some degree of performance sacrifice maybe acceptable.

It is contemplated that the embodiments described herein may be utilizedfor purposes other than those described and that the challenges that maybe noted herein are not intended to be an exhaustive list of challengesthat may be overcome by the embodiments of the present invention. Suchchallenges that may be described herein are noted for illustrativepurposes and that all of the challenges that may be overcome by thevarious embodiments of the present invention are not described forpurposes of brevity. Moreover, it is contemplated that the embodimentsdescribed herein may provide for any number of advantages and that thosenoted are not intended to be an exhaustive list that may be achieved.Such advantages disclosed herein are noted for illustrative purposes andthat all of the advantages achieved by the embodiments of the presentinvention are not described for purposes of brevity as well.Furthermore, the examples provided herein are disclosed for illustrativepurposes and are not intended to be an exhaustive list of examples thatare capable of being implemented and are not intended to limit the scopeof the embodiments of the present invention in any manner.

The embodiments of the present invention as set forth in FIGS. 3-14generally illustrate and describe a plurality of circuits or otherelectrical devices. All references to the circuits and other electricaldevices and the functionality provided by each, are not intended to belimited to encompassing only what is illustrated and described herein.While particular labels may be assigned to the various circuits or otherelectrical devices disclosed, such labels are not intended to limit thescope of operation for the circuits and the other electrical devices.Such circuits and other electrical devices may be combined with eachother and/or separated in any manner based on the particular type ofelectrical implementation that is desired in the camera. It isrecognized that any circuit or other electrical device disclosed hereinmay include any number of microprocessors, integrated circuits, memorydevices (e.g., FLASH, RAM, ROM, EPROM, EEPROM, or other suitablevariants thereof) and software which co-act with one another to generatethe oscillating signals to perform analog-to-digital conversion of arrayoutputs and to demodulate the array outputs.

FIG. 1 depicts a conventional microbolometer based detector 20. Thedetector 20 may be implemented within a camera. The detector 20 maycomprise a plurality of pixels 22 that are arranged in 320×240 array(e.g., 320 columns and 240 rows). Each pixel 22 includes amicrobolometer 24, and a switch 28. The switch 28 may be implemented asa field effect transistor (FET). It is known that the microbolometers 24and the switches 28, are formed on a semiconductor substrate. Thedetector 20 may be implemented with a pixel pitch of 45 um using a 3.3V0.5 um Complementary Metal-Oxide Semiconductor (CMOS) technology.

A selectable DC based power supply (not shown) closes the switches 28 insequence, row by row (e.g., all switches in a row are closed at the sametime while all other switches in different rows are open) so thatcurrent from one microbolometer 24 in a column flows therefrom. Thecondition of measuring a single bolometer in a time slice that is 1/N(where N corresponds to the number of rows) before the cycle repeats isgenerally defined as time division multiplexing (TDM).

A capacitive trans-impedance amplifier (CTIA) 30 is coupled to theoutput of each pixel 22 for a given column. A capacitor 32 is coupled toeach CTIA 30. The size of the capacitor 32 controls the gain of the CTIA30 output. Each CTIA 30 performs a current-voltage conversion byintegrating a charge on the capacitor 32. A switch 34 may serve to resetthe current to voltage conversion performed by the CTIA 30.

A switch 36 and capacitor 42 are coupled to an output of the CTIA 30 toperform a sample and hold (S&H) operation for a given column. When theproper amount of charge is integrated across the capacitor 32, theswitch 36 closes momentarily to transfer the charge to the capacitor 42.The purpose of S&H operation is to hold the charge collected from thecapacitor 32 to await digitization.

An additional amplifier 38 and switch 40 is provided so that the outputfrom each column can be read. The switch 40 can be configured to closeto enable the output for corresponding column to pass through amultiplexer. Once the output for a given column is ascertained, theswitches 28 and 40 are opened and the switches 28 and 40 for a precedingrow are closed so that a reading for such a row can be ascertained. Thissequence occurs one at time for every row within the array. As notedabove, the detector 20 employs a TDM approach such that the FET switch28 for a given row is closed one at a time so that the correspondingoutput for the given row is ascertained. The outputs for each column aretransmitted on a signal VIDEO_OUTPUT to an Analog to Digital (A/D)converter (not shown). The detector 20 as used in connection with theTDM approach may exhibit noise aliasing. Noise aliasing may change theshape of the noise spectrum (e.g., noise exhibits non-linear patterns)and become less predictable. As such, if one cannot predict the noise,then design options may be not able to be used that may otherwiseeliminate the effects of such predictable noise to increase cameraperformance.

FIG. 2 depicts a conventional thermopile based detector 50. The detector50 may be implemented within a camera. The detector 50 is generallypackaged, mounted on a circuit board and enclosed by a cap in which alens is arranged. The detector 50 includes a plurality of pixels 52 thatmay be arranged in 320×240 array (e.g., 320 columns and 240 rows). Eachpixel 52 includes a thermopile sensor element 54 and a switch 56. Theswitch 56 is implemented as a FET.

A column decoder 58 is provided and includes a DC power supply thatselectively closes the switches 56 on a column wise bases, one column ata time (i.e., the detector 50 employs a TDM scheme). Each thermopile 54in the corresponding column generates an output voltage in response tothe switch 56 being closed. A low noise amplifier 60 is operably coupledto each thermopile 54 in a given row. The amplifier 60 is generallyconfigured to provide a higher output gain than that of the amplifierused in connection with the detector 20 (e.g., the microbolometer baseddetector). A representative amplifier that may be used for increasingthe gain from the thermopiles 54 is an LT6014 that is provided by LinearTechnology of 1630 McCarthy Blvd., Milpitas, Calif. 95035-7417. A lead62 is provided for distributing the output voltage from the thermopile54 to a device that is not included within the detector 50. Theamplifier 56 increases the output voltage provided from the thermopile54.

In general, after each thermopile 54 within a given column is enabled bya corresponding FET switch 56, each amplifier 60 that is coupled to thethermopile 54 requires a settling time. After such a settling time isachieved, the voltage output provided by the thermopile 54 is digitizedso that the image can be rendered as an electronic image.

It is known that thermopiles generally have a good signal-to-noiseratio. It is also known that thermopiles generally exhibit a lowresponse and low noise. In order to increase the response, the low noiseamplifier 60 may be needed to increase the gain for a particular row ofpixels 52. However, the use of such low noise amplifiers may still add asignificant amount of noise in the detector 50 readout. Particularly,for amplifiers that are incorporated on the same silicon substrate asthe detector 50. The detector 50 may also experience noise aliasing. Asnoted above, such a condition may reduce the ability to predict noise.If the noise cannot be predicted, then it may not be possible toimplement design options that may mitigate the effects of predictablenoise patterns.

FIG. 3 depicts a thermopile IR detector 70 in accordance to oneembodiment of the present invention. The detector 70 may be implementedwithin an imaging device 69 such as, but not limited to, a camera. Thedetector 70 is generally put into a package and mounted on a circuitboard 71. The detector 70 and the circuit board 71 are enclosed by a cap73 in which a lens 74 is arranged. The detector 70 generally comprises afunction generator 72 and a thermopile array 76 or 76′. The functiongenerator 72 may drive each column (or row) of the thermopiles at thesame time with an oscillating carrier (or oscillating signal). Eachthermopile generates an electrical output in response to the thermalenergy captured from the object. The corresponding electrical outputthat is generated by the thermopile is amplitude modulated with theoscillating carrier signal and transmitted therefrom. Each column (orrow) of thermopiles is driven at a unique frequency from one another. Ingeneral, the function generator 72 is configured to activate all of thethermopiles in all of the columns (or rows) to amplitude modulate theoutput from each thermopile (e.g., through the use of one or moreswitches that may be coupled to each thermopile) with the oscillatingcarrier which is at a unique frequency for each column (or row). All ofthe thermopiles may be active at the same time. A gain circuit 78 thatincludes a plurality of amplifiers is operably coupled to the thermopilearray 76 or 76′. Each amplifier is coupled to a particular column (orrow) of thermopiles to increase the signal strength for each column (orrow) of thermopiles. A demodulation circuit 84 is generally coupled tothe gain circuit 78 and is configured to separate the orthogonalcarriers for each column (or row) of thermopiles so that thecorresponding voltage (or current) output from each column ofthermopiles can be ascertained in order to generate an electronic imageof the captured original image. It is contemplated that the embodimentsof the present invention may utilize frequency modulation or phasemodulation. Any reference to a thermopile being in a row may also applyto such thermopile being in a column.

The concept of modulating all of the thermopiles for all columns (orrows) with an oscillating carrier at a unique frequency for each columnin which all of the carriers are simultaneously presented to each column(or row) and modulated within an array is generally defined as aFrequency Division Multiplexing (FDM) approach. The FDM approach enablesthe use of a dedicated amplifier to be added to every row in thethermopile array 76 or array 76′ to increase the signal strengthirrespective of the amount of noise generated by such amplifier. Forexample, a natural consequence of amplitude modulating each of thethermopiles for a given column (or row) with a unique carrier signal ata predetermined frequency and then simultaneously presenting suchsignals to the amplifiers with the gain circuit 78 is that the broadbandnoise of the channel becomes large (e.g., a standard deviation of thebroadband noise grows by the square root of the number of thermopiles onthe column (or row)). If the broadband channel noise is “large” comparedto the broadband amplifier noise, the broadband noise created by theamplifier on the given column (or row) becomes insignificant due to thefact that the broadband noise for both the channel and the amplifieradds up as a quadrature sum (e.g., square root of the sum of squares ofthe noise standard deviations) so the amount of noise introduced by theelectronics is considered to be inconsequential.

It is also contemplated that the materials used to construct thethermopiles in the array 76 or 76′ may comprise compounds in the(Bi1-xSbx)2 (Te1-ySey)3 family (e.g., Bismuth-Tellurium family). Thefamily of compounds will be denoted by Bi2Te3 for brevity. The use ofBi2Te3 to construct the thermopiles in the array 76 or 76′ may cause thethermopile resistance to fall below 10 K Ohms, which can cause adecrease in the amount of thermopile (or detector) noise. While Bi2Te3based materials can be used to construct thermopiles for the TDMapproach to reduce thermopile noise, such a reduction in noise may beminimized when compared to the amount of noise created by the amplifier(e.g., see amplifier 60 in FIG. 2). The large amount of noise created bythe amplifier may be mitigated due in large part to the implementationof the FDM approach for the reasons noted above.

In general, the use of Bi2Te3 may produce a very high performancethermopile based detector if the amplifier was ideal with no noise.Because the impedance (or resistance) of a Bi2Te3 based thermopile is solow, its noise is also low. To read out a low impedance thermopile andnot add any noise to the output signal may require a very low noiseamplifier. This may be an issue with the TDM approach as it may benecessary to read out a high performance thermopile with a very highperformance amplifier. High performance may mean high power because thenoise from the amplifier is reduced the more power the input stage ofthe amplifier consumes. On the other hand, the FDM approach mayincorporate low impedance (e.g., high performance) thermopiles that arein series (see FIG. 5) to increase the overall noise presented to theamplifier. Since the total noise standard deviation is computed by thesquare root of the sum of the squares of the thermopile standarddeviation (all in parallel or series (see FIGS. 5 and 8)) and theamplifier standard deviation, the total noise may be primarily dominatedby the noise from the thermopiles. While the overall signal beforedemodulation may be noisy, such a noisy signal may be averaged (e.g., byintegrating) over a much longer time (e.g., the image frame rate time).Because the overall signal can be integrated over this longer period oftime, the signal can be built back to the noise ratio of a singlethermopile detector close to its original value after demodulation andthe influence of the amplifier noise can be shown to nearly vanish. Thiscondition may illustrate the notion of predicting the noise and usingmeasures within the design to eliminate its effects.

It is further recognized that the materials used to construct thethermopile in the array 76 or 76′ may comprise superlattice quantum wellmaterials as set forth in co-pending application Serial No.PCT/US2011/55220, (“the '520 application”), entitled “SUPERLATTICEQUANTUM WELL INFRARED DETECTOR” filed on Oct. 7, 2011, which is herebyincorporated by reference in its entirety.

The following illustrates the manner in which the FDM approach mayreduce the electronic noise in comparison to the TDM approach. Inparticular, the signal to noise (SNR) ratio will be computed for the TDMapproach and the FDM approach. The signal from a ith detector(thermopile) under TDM can be written as:

r _(i)(t)=v _(si) +n _(d)(t)+n _(e)(t)  (1)

where:r_(i)(t)=Received signal from i_(th) detectorv_(si)=Signal voltage from i_(th) detector (V)n_(d)(t)=zero-mean white Gaussian detector noise with spectral height

${\overset{.}{v}}_{d}^{2}\left( \frac{V^{2}}{H\; z} \right)$

n_(e)(t)=zero-mean white Gaussian electronics noise with spectral height

${\overset{.}{v}}_{e}^{2}\left( \frac{V^{2}}{H\; z} \right)$E[n _(d)(t)n _(e)(t)]=0

E[·]=statistical expectationVar[·]=statistical variance

In the TDM approach, the detector is sampled for a fraction of the frametime, T_(frame). The fraction of time is determined based on the numberof detectors in a row that need to be multiplexed out, N_(column). Theoutput of a standard integrator is:

$\begin{matrix}{V_{TDM} = {\int_{0}^{\frac{T_{frame}}{N_{column}}}{{r_{i}(t)}{t}}}} & (2)\end{matrix}$

The SNR is given by the following equation:

$\begin{matrix}{{SNR}_{TDM}^{2} = \frac{\int_{\; 0}^{\frac{T_{frame}}{N_{column}}}{{E\left\lbrack {r_{i}(t)} \right\rbrack}^{2}{t}}}{{Var}\left\lbrack V_{TDM} \right\rbrack}} & (3)\end{matrix}$

The SNR for TDM can now be evaluated:

$\begin{matrix}{{SNR}_{TDM}^{2} = \frac{v_{s_{i}}^{2} \cdot T_{frame}}{N_{column}\left( {v_{d}^{2} + v_{e}^{2}} \right)}} & (4)\end{matrix}$

For the FDM approach, each detector is modulated on a unique orthogonalcarrier, si(t). It will be shown later that for the FDM approach, all ofthe detectors are present all the time on the row bus. The consequenceof this is that the noise variance of each detector are added together.The signal on the row bus becomes:

r(t)=Σ_(u=1) ^(N) ^(columns) [v _(s) _(i) s _(i)(t)]+n _(d)′(t)+n_(e)(t)  (5)

where:r(t)=Received signals_(i)(t)=Orthogonal carrier iv_(s) _(i) =Thermopile signal or orthogonal carrier i (V)n_(d)′(t)=zero-mean white Gaussian detector noise with spectral height

$N_{columns} \cdot {v_{d}^{2}\left( \frac{V^{2}}{H\; z} \right)}$

n_(e)′(t)=zero-mean white Gaussian electronics noise with spectralheight

$\begin{matrix}{{v_{e}^{2}\left( \frac{V^{2}}{H\; z} \right)}{{E\left\lbrack {{n_{d}(t)}{n_{e}(t)}} \right\rbrack} = 0}{and}\left\{ \begin{matrix}{{\int_{0}^{T_{frame}}{{s_{i}(t)}{s_{j}(t)}{t}}} = {{T_{frame}\mspace{14mu} {for}\mspace{14mu} i} = j}} \\{{\int_{0}^{T_{frame}}{{s_{i}(t)}{s_{j}(t)}{t}}} = {{0\mspace{14mu} {for}\mspace{14mu} i} \neq j}}\end{matrix} \right.} & (6)\end{matrix}$

In FDM approach, the detector is sampled for the full frame time,T_(frame) because all the detectors are on all the time. The output of astandard integrator is:

V _(FDM)=∫₀ ^(T) ^(frame) r(t)s _(i)(t)dt  (7)

The SNR for FDM can now be evaluated for the i^(th) component:

$\begin{matrix}\left\{ \begin{matrix}\begin{matrix}{{SNR}_{FDM}^{2} = \frac{\int_{0}^{T_{frame}}{{E\left\lbrack {r_{i}(t)} \right\rbrack}^{2}{t}}}{{Var}\left\lbrack V_{FDM} \right\rbrack}} \\{\mspace{59mu} {= \frac{v_{s_{i}}^{2} \cdot T_{frame}}{\left( {{N_{column}v_{d}^{2}}\; + v_{e}^{2}} \right)}}}\end{matrix} \\{\mspace{121mu} {= \frac{v_{s_{i}}^{2} \cdot T_{frame}}{N_{column}\left( {v_{d}^{2} + \frac{v_{e}^{2}}{N_{column}}} \right)}}}\end{matrix} \right. & (8)\end{matrix}$

Comparing Equation 8 to Equation 4, it can be seen that with the FDMapproach, the electronic noise variance decreases based on the number ofdetectors that are multiplexed out (e.g., N_(column).).

In general, it is recognized that the oscillating carriers may includeany orthogonal set of functions such as, but not limited to, WalshFunctions, sine and cosine functions.

The Walsh functions as used herein may be denoted by wal(0, θ), sal(i,θ), sal(i, θ)′, cal(i, θ), and/or cal(i, θ)′ (where θ is normalized timet/T). Walsh functions may generally form a complete system oforthonormal functions, which may be similar to the system of sine andcosine functions. There is a close connection between sal and sinefunctions, as well as between cal and cosine functions. In general,Walsh functions are known to form a complete orthonormal set and aretherefore orthogonal.

FIG. 4 depicts a function generator 72 implemented within the detector70 of FIG. 3 in accordance to one embodiment of the present invention.The function generator 72 is configured to generate Walsh functions suchas sal(x, t) and cal(y, t). For example, the function generator 72generates the functions sal(1, t) through sal(8, t) and cal(1, t)through cal(8, t). In one example, the function generator 72 may be a4-bit synchronous counter. It is recognized that the function generator72 may be configured to accommodate for any number of bits and that thenumber of bits selected generally depends on the size (e.g., number ofcolumns and/or rows) of the thermopile array. In addition, it is furtherrecognized that the function generator 72 may be non-synchronous.

The function generator 72 includes a plurality of exclusive-or (XOR)gates 86 for receiving one or more bits (e.g., 4 bits) to generate thefunctions sal(1, t)-sal(8, t) and the functions cal(1, t)-cal (7, t) Ingeneral, the arrangement of the XOR gates 86 and the clock areconfigured such that each function of sal (x, t) and cal (y, t) istransmitted at a different period from one another so that apredetermined frequency is maintained between each function of sal(x, t)and cal(y, t). Each function of sal(x,t) and cal(y,t) is transmitted toa different column within the array 76. For example, sal(1,t) andcal(1,t) may be transmitted to a first column of thermopiles within thearray and so on, in which sal(8,t) and cal(8,t) are transmitted to aneight column within the array 76. Because each function of sal (x, t)and cal (y, t) may be transmitted at a different period from one anotherto maintain a predetermined frequency therebetween, such a condition mayensure that every column of thermopiles are modulated by the orthogonalset (e.g., of sal and/or cal functions) at a unique frequency.

It is recognized that the function generator 72 may be modified orchanged to provide any number of Walsh functions (e.g., sal (x, t), sal(x, t)′, cal (y, t), or cal (y, t)′). The particular implementation ofthe function generator 72 may be modified to provide any sequence of sal(x, t), sal (x, t)′, cal (y, t), or cal (y, t)′ (one or more of thesefunctions (or any combination thereof may be referred to hereafter asWalsh Function(s) or Walsh (x, t), Walsh (x, t)′, Walsh (y, t), or Walsh(y, t)′, etc.)) to the array 76 or 76′ based on the desired criteria ofa particular implementation.

FIG. 5 depicts the thermopile array 76 implemented within the detector70 of FIG. 3 in accordance to one embodiment of the present invention.The array 76 of FIG. 5 is shown in a voltage summing configuration. Thearray 76 includes a plurality of pixels 90 (or thermal sensing elements)that are arranged in a 8×N array. For example, the array 76 includes 8columns of pixels 90 and any number of rows of pixels 90. Each pixel 90includes a first pair of switches 92, a second pair of switches 94, athermopile 96, and a switch 98. It is recognized that the quantity ofswitches and thermopiles within each pixel may vary based on the desiredcriteria of a particular implementation. The switches 92 and 94 maycoact with the thermopile to modulate the output of thermopile onto theoscillating signals. The columns of pixels 90 are configured to receivethe functions sal(1,t)-sal(8, t); and cal(1, t)-cal(8, t) from thefunction generator 72. For example, pixel 90 in column 1 receives thefunction sal(1, t) and cal(1,t); pixel 90 in column 2 receives thefunctions sal(2,t) and cal(2, t) and so on such that the pixel in column8 receives the functions sal(8,t) and cal(8,t). The Walsh functions asshown in FIG. 5 are examples. Different arrangements of the Walshfunctions may be presented to the array 76.

It is recognized that the size of the array may vary and that the numberof columns and rows may be selected based on the desired criteria of aparticular implementation. It is also recognized that the number andconfiguration of switches 92, 94 may vary based on the desired criteriaof a particular implementation. The use of such functions may vary aswell based on the desired criteria of a particular implementation. Thecircuit as depicted within the array 76 (or elsewhere in the detector70) is used for illustrative purposes and is not intended to demonstratethat the embodiments of the present invention are to be implemented inthis manner alone.

As noted above, each Walsh function is transmitted at a unique frequencyto each corresponding column of pixels 90 (e.g., column 1 receives afirst Walsh function and a second Walsh function at a first frequency,column 2 receives third Walsh function and a fourth Walsh function at asecond frequency, column 3 receives a fifth Walsh function and a sixthWalsh function at a third frequency and so on). Each unique frequencymay be separated by a predetermined amount to ensure that an outputsignal from each pixel 90 can be uniquely recovered during demodulation.In one example, the separation frequency may be 30 Hz.

In general, each column of pixels 90 is driven with the Walsh functionsand operate at a unique frequency from one another such that a voltageoutput from each thermopile 96 is read out on a row-wise basis. Whileeach thermopile 96 may be a particular Walsh function, half of thethermopiles 96 on a corresponding row may be in forward direction (+sideon row bus) and the other half of the thermopiles 96 may be in thereverse direction (−side on row bus) due to the cyclical nature of theorthogonal carriers (e.g, the Walsh functions). It is recognized thatthe voltage output from a given row is usually near ground because halfof the thermopiles 96 may be in the forward direction while theremaining half of the thermopiles 96 are in the reverse direction. Theoverall dynamic range (e.g., the ratio of the highest measurable signalto the lowest measurable signal) is maintained. The Walsh functionsprovide for non-overlapping clocks for each pixel 90, which enablessuitable switching for each pixel 90.

The array 76 transmits the voltage output for each row on the signalv₁(t) through v_(n)(t) (where N=the number of rows in the array). A gaincircuit 78 includes a plurality of amplifiers 102 that receives thevoltage outputs v₁(t)-v_(n)(t) and increases the amplitude for such togenerate the voltage outputs v₁′(t)-v_(n)′(t). In one example, eachamplifier 102 may be a CMOS amplifier similar to LMC6022 from NationalSemiconductor of 2900 Semiconductor Drive, Santa Clara, Calif. 95052.Each amplifier 102 may be integrated on the same silicon substrate asthe array 76. It is recognized that the type of amplifier used may varybased on the desired criteria of a particular implementation. As notedabove in connection with FIG. 2, thermopiles generally exhibit a lowresponse and require additional gain to increase the output. Thethermopiles 96 are connected in series with one another in a given rowand the corresponding voltage output is presented to the non-invertinginput of the amplifier 102. Due to such an arrangement, the switch 98 isadded across each thermopile 96 to permanently close its correspondingpixel in the event the thermopile 96 is damaged. The coupling of thethermopiles 96 in series in a particular row and the presentation of thevoltage output form that row to the non-inverting input of the amplifier102 increases the gain voltage output and reduces the potential for 1/fnoise because of the small current flow into the non-inverting input ofthe amplifier 102.

Referring to FIGS. 3 and 5, a multiplexer 80 receives the outputvoltages v₁′(t)-v_(n)′(t) from the gain circuit 78. An analog to digital(A/D) converter 82 receives an output voltage v₁′(t)-v_(n)′(t) over asingle wire bus. The A/D converter 82 converts the output voltagev₁′(t)-v_(n)′(t) from an analog voltage signal into a digital voltagesignal. The A/D converter 82 may include any combination of hardware andsoftware that enables analog to digital conversion.

The demodulation circuit 84 is configured to receive a digital outputfrom the A/D converter 82 for each row in the array 76. The demodulationcircuit 84 may be a matched filter, a Fast Walsh Transform or any othersuitable circuit that includes any combination of hardware and softwareto determine the voltage output for a given row of thermopiles 96 in thearray 76. The output from the A/D converter 82 comprises a digitalrepresentation of the output voltage from a row of thermopiles 96 thatis in the form of a constant that is multiplied to the correspondingorthogonal carriers (e.g., the Walsh functions that are transmitted atthe unique frequency for each column).

Each of the unique orthogonal carriers includes the thermopile signalinformation. Multiplying the received signal by sal(i, t) (or cal(i,t)-if cal (i, t) is used, only a sign change will occur) performs thedemodulation. The demodulated signal is then averaged to estimate thethermopile signal. The received signal from a row is given by Equation9:

$\begin{matrix}{{r(t)} = {{\sum\limits_{i = 1}^{N_{column}}\left\lbrack {{m(t)}{{sal}\left( {i,t} \right)}} \right\rbrack} + {v_{n}(t)}}} & (9)\end{matrix}$

Depending on the scene and thermal time constant, m(t) can be consideredto be either a constant or a random variable to be estimated. Assumingthat the parameter to be estimated is a constant, the optimal estimatoris given by:

$\begin{matrix}{{\overset{}{m}}_{i} = {\frac{1}{T_{frame}}{\int_{0}^{T_{frame}}{{r(t)}{{sal}\left( {i,t} \right)}{t}}}}} & (10)\end{matrix}$

where:{circumflex over (m)}_(t)=Estimated thermopile output signal from thei^(th) detectorSince sal(i, t) is either +1 or −1 implementation in a digital signalprocessor (DSP) or field-programmable gate array (FPGA) may be simple.

FIG. 6 depicts a thermopile array 76′ implemented with the IR detectorof FIG. 3 in accordance to another embodiment of the present invention.The array 76′ of FIG. 5 is shown in a current summing configuration. Thearray 76′ includes the plurality of pixels 90 (or thermal sensingelements) that are arranged in an 8×N array. Each of the thermopiles 90is in parallel with one another. The array 76′ includes 8 columns ofpixels 90 and any number of rows of pixels 90. Each pixel 90 includesthe first pair of switches 92, the second pair of switches 94, thethermopile 96, and a switch 98 (or a safety switch). It is recognizedthat the number of switches and thermopiles within each pixel may varybased on the desired criteria of a particular implementation. In asimilar manner to that discussed above in connection with FIG. 5, theswitches 92 and 94 may coact with the thermopile to modulate the outputof thermopile onto the oscillating signals. The columns of pixels 90 areconfigured to receive the Walsh functions from a function generator. Forexample, pixel 90 in column 1 receives a first Walsh function and asecond Walsh function; pixel 90 in column 2 receives a third Walshfunction and a fourth Walsh function and so on.

It is recognized that the size of the array may vary and that the numberof columns and rows may be selected based on the desired criteria of aparticular implementation. It is also recognized that the number andconfiguration of switches 92, 94 may vary based on the desired criteriaof a particular implementation. As noted above, each Walsh function istransmitted at a unique frequency to each corresponding column of pixels90 (e.g., column 1 receives a first Walsh function and a second Walshfunction at a first frequency, column 2 receives third Walsh functionand a fourth Walsh function at a second frequency, column 3 receives afifth Walsh function and a sixth Walsh function at a third frequency andso on). Each unique frequency may be separated by a predetermined amountto ensure that an output signal from each pixel 90 can be uniquelyrecovered during demodulation. In one example, the separation frequencymay be 30 Hz.

Similar to the operation noted in the array 76 (e.g., the voltagesumming configuration), each column of pixels 90 in the array 76′ isdriven by the Walsh functions and operates at a unique frequency fromone another such that a current output from each thermopile 96 is readout on a row-wise basis. While each thermopile 96 may be a particularWalsh function, half of the thermopiles 96 on a corresponding row may bein forward direction (+side on row bus) and the other half of thethermopiles 96 may be in the reverse direction (−side on row bus) due tothe cyclical nature of the orthogonal carriers (e.g, sal (x, t), sal (x,t)′, cal (y, t), or cal (y, t)′). It is recognized that the currentoutput from a given row is usually near ground because half of thethermopiles 96 may be in the forward direction while the remaining halfof the thermopiles 96 are in the reverse direction. The overall dynamicrange (e.g., the ratio of the highest measurable signal to the lowestmeasurable signal) is maintained. The Walsh functions provide fornon-overlapping clocks for each pixel 90, such a condition enablessuitable switching for each pixel 90.

The thermopiles 90 in the rows (or columns) may each provide a modulatedcurrent output that is indicative of the sensed temperature from thescene. The array 76′ transmits the current output for each row on thesignal I₁(t) through I_(n)(t). The gain circuit 78 includes theplurality of amplifiers 102 that receives the current outputsI₁(t)-I_(n) (t) and converts/increases the amplitude for such togenerate the voltage outputs V₁′(t)-V_(n)′(t). Each amplifier 102 may beintegrated on the same silicon substrate as the array 76 or 76′. Asnoted above in connection with FIG. 2, thermopiles generally exhibit alow response and require additional gain to increase the output. Thethermopiles 96 are connected in parallel with one another in a given rowand the corresponding current output is presented to the inverting inputof the amplifier 102. The switch 98 is added at an output of thethermopile 96 and in its normal state, is in a closed position to enablecurrent to flow all of the thermopiles 96 in a row (or column) on to thegain circuit 78. In the event the thermopile 96 is damaged, the switch98 opens to remove the damaged thermopile 96 from the string ofthermopiles 96 on a given row or column to enable the remainingthermopiles 96 (on the same column or row) to continue to providecurrent to the gain circuit 78. It is recognized that with the currentsumming configuration that the noise attributed to the amplifier andother electronics may be reduced as well. While the coupling of thethermopiles 96 in parallel in a particular row and the presentation ofthe current output from that row to the inverting input of the amplifier102 may exhibit an increase which may be much greater than the inputnoise of the amplifier, the detector signal to noise ratio may berecovered via the longer integration time using FDM and thusdramatically reduce the influence of the amplifier noise (and/or otherelectronic noise not only from the amplifiers in the gain circuit 78 butelsewhere prior to demodulation).

The operation of the multiplexer 80, the A/D converter 82, and thedemodulation circuit 84 as noted used in connection with the array 76′is similar to that described above for the array 76.

FIG. 7 depicts a thermopile IR detector 150 in accordance to anotherembodiment of the present invention. The detector 150 includes aplurality of oscillators 152 (or function generator), an array 154, again circuit 156, a multiplexer circuit 158, an A/D converter 160, amemory circuit 162, and a demodulation circuit 164. The plurality ofoscillators 152 is configured to generate oscillating carrier signals ata predetermined frequency for activating all thermopiles within a givencolumn (or row) so that modulated signals are transmitted therefrom. Forexample, each oscillator 152 is configured to generate an oscillatingsignal at a unique frequency and to transmit the same to a correspondingcolumn of pixels within the array 154. Each of the columns of pixels isdriven at the same time but at different frequency from one another. Thedetector 150 employs the FDM approach as noted in connection with FIG.3.

The plurality of oscillators 152 is voltage controlled via a voltagesource 166. It is contemplated that different types of oscillators maybe used instead of a voltage-controlled oscillator. For example, suchoscillators may be coupled to a mechanical resonator (such as, but notlimited to, a crystal). The type of device used to generate theoscillating signal at the unique frequency may vary based on the desiredcriteria of a particular implementation. A plurality of resistors 155 ispositioned between the oscillators 152 and the voltage source 166 toadjust the voltage output of the voltage source. The resistance valuefor each resistor 155 may be selected to ensure such that a differentvoltage input is provided to each oscillator 152. Such a condition mayensure that the oscillators 152 generate a unique frequency from oneanother in the event the oscillators 152 are voltage controlled. Theoscillators 72 each generate an oscillating signal that is in the formof a sine function (e.g., sin (x, t)) or a cosine function (e.g., cos(y,t)).

FIG. 8 depicts a more detailed diagram of the thermopile array 154. Thearray 154 is also shown in a current summing configuration. The array154 includes pixels 202 (or thermal sensing elements) that are arrangedin an M×N array. Each pixel 202 includes a thermopile 204 and a FETbased switch 206. The number of thermopiles and switches implementedwithin a given pixel may vary based on the desired criteria of aparticular implementation. All of the oscillators 152 are active all ofthe time such that all of the columns of pixels are amplitude modulatedwith a unique frequency. For example, the thermopiles 204 in column 1are driven by a first oscillating signal at a first frequency and thethermopiles 204 in column M are driven by a second oscillating signal ata second frequency, where first frequency is different from the secondfrequency. In one example, the first frequency may be 30 Hz and thesecond frequency may be 60 Hz. The particular frequency used for eachcolumn is generally defined by:

f(i)=i*30Hz,  (11)

-   -   where i corresponds to the column number.

It is recognized that metal film bolometers (or low resistancebolometers) may be implemented instead of the thermopiles with the FDMapproach.

As noted in connection with FIGS. 3 and 5, each oscillator 152 isgenerally configured to activate all of the thermopiles for acorresponding column (or row) with an amplitude modulated orthogonalcarrier at a unique frequency so that all of the thermopiles in such acolumn (or row) are on for the entire frame time. This may be performedfor all columns within the array 154. As such, it can be said that allof the thermopiles within the array 154 are active at the same time.

An amplifier 208 may increase the voltage output (or current output) foreach row. The multiplexer circuit 158 transmits each voltage output froma row on a single line to the A/D converter 160. The A/D converter 160converts the voltage output into a digital based output. The A/Dconverter 160 may include any combination of hardware and software toperform the conversion. A memory circuit 162 stores the digitalizedoutput to enable transfer to the demodulation circuit 164. In oneexample, the memory circuit 162 may be implemented as a Direct MemoryAccess (DMA) storage device or other suitable storage mechanism. Thedemodulation circuit 164 performs a Fast Fourier Transform (FFT) on thedigitized output. The demodulation circuit 164 may include anycombination of hardware and software to perform the FFT. An image resultdepicting the captured image is generated therefrom.

It is recognized that thermopile based arrays within the detectors 70and 150 (or other suitable variants thereof) may exhibit increasedlevels of thermal stability and thus may be easy to maintain radiometriccalibration over a wide range of ambient temperatures. It is alsorecognized that thermopile based arrays within the detectors 70 and 150(or other suitable variants thereof) that utilize the FDM approach maybe adaptable for a range of capabilities such as, but not limited to,fire fighting applications as such an array may not require specialimage processing techniques (e.g., combining higher noise low gainimages with lower noise high gain images) to display images with bothhot and cold objects in the capture image. It is also recognized thatthermopile based arrays within the detectors 70 and 150 (or othersuitable variants thereof) may respond linearly to incoming radiancefrom an object. Due to such a linear response, a low cost in-factoryradiometric calibration may be achieved. It is also recognized thatthermopile output based signals from the thermopiles within thedetectors 70 and 150 (or other suitable variants thereof) are generallydifferential and unbiased and may not exhibit large drift offsets. Assuch, radiometric calibration may be easier to maintain over a widerange of ambient temperature. It is also recognized that that thedetectors 70 and 150 (or other suitable variants thereof) when used in avoltage summing configuration may not exhibit 1/f noise due to the FDMapproach, which nearly eliminates the 1/f noise from the amplifier(and/or from additional electronics in the detector) by modulating theoutput of the thermopile at a high enough frequency where the 1/f noiseof the amplifier is negligible. It is also recognized that the detectors70 and 150 (or other suitable variants thereof) may be able to capture,but not limited to, short temporal events because all of the thermopileswithin the array may be capturing energy all of the time.

FIG. 9 depicts a thermopile 96 and a switching arrangement 220 for thevoltage summing configuration of the array 76 in accordance to oneembodiment of the present invention. The pixel 90 as depicted in FIG. 9is generally indicative of a basic chopper modulated (un-balanced)pixel. Because the pixel 90 is a basic chopper modulated pixel, it isactive only half of the time.

In general, the following FIGS. 9-14 depict various switchingarrangements 220 that may be used in connection with removing a damagedthermopile 96 from a row or column of thermopiles 96 such that theoperating thermopiles are free to continue to provide a modulatedelectrical output therefrom. These figures depict ways in which thedamaged thermopile may be bypassed to enable the remaining thermopilesto provide the modulated electrical output.

The switching arrangement 220 includes first logic circuit 222, a secondlogic circuit 224, and a first switch 226. In one example, the firstswitch 226 may be implemented as an N-channel MOSFET. In anotherexample, the switches not generally used in an active modulation schememay be a polysilicon fuse. It is recognized that particular type ofswitch as disclosed herein may vary based on the desired criteria of aparticular implementation.

The first switch 226 may be used to modulate the electric output fromthe thermopile 96 and may also serve as a safety (or bypass) switch inthe event the thermopile is damaged and exhibits a short or opencondition due to failure. A memory cell 260 provides data (e.g., binarydata (low output “0” or high output “1”)) to the first logic circuit222. The function generator 72 provides a Walsh functions (e.g., sal (j,t), sal (j, t)′, cal (k,t), and/or cal (k, t)′) to the second logiccircuit 224.

For normal operation of the thermopile 96, it may be desirable to allowthe Walsh function to modulate on the electrical output of the pixel 90.As such, the memory cell provides high output (e.g., “1”) to the firstlogic circuit 222. The first logic circuit 222 generates low output(e.g., “0’) in response thereto and the second logic circuit 224generates high output when the Walsh function exhibits a high output.The first switch 226 is closed enabling the thermopile 96 to provide themodulated electrical output to the next pixel or to the input of theamplifier 102. When the Walsh function exhibits low output, no output isprovided by thermopile 96, however a modulated electrical voltage from aprevious pixel(s) may be passed through the thermopile 96.

When the thermopile 96 is damaged, it may be desirable in this case toallow the modulated electrical output from the previous pixel to passthrough the thermopile to provide the modulated electrical output to thenext pixel or to be input of the amplifier 102. If one thermopile 96 ina series of thermopiles 96 in a row or column are damaged in the voltagesumming configuration, then such a condition may take out the entireseries of thermopiles 96 in the row (or column). Accordingly, the switch226 serves as a safety bypass. If a particular thermopile 96 is damaged,it is necessary to allow the remaining pixels in the row (column) toprovide an output. To account for this condition, the memory cell 260outputs low output to the first logic circuit 222 when thermopile 96 isdetected to be damaged. The first logic circuit 222 generates highoutput. The second logic circuit 224 provides high output to close thefirst switch 226. The modulated electrical output from the previouspixel 90 may pass through the first switch 226 and around the damagedthermopile 96 (or bypasses the damaged thermopile 96).

For all noted switching implementations as noted herein, each detector10 may enable diagnostics such that it is possible to determine whichpixel 90 in the array 76, 76′ is damaged. This condition may beperformed when the detector 10 is manufactured. For example, after thedetector 10 is manufactured, a diagnostic test may be performed toidentify which thermopile(s) are damaged. A bad pixel map is generatedand populated with data corresponding to the damaged thermopile(s). Thevarious bypass or safety switch can be closed (i.e., for a voltagesumming configuration) or opened (i.e., for a current summingconfiguration) to enable the remaining thermopiles in row or column toprovide a modulated electrical output.

FIG. 10 depicts a thermopile 96 and a switching arrangement 220 for thevoltage summing configuration of the array 76 in accordance to oneembodiment of the present invention. The pixel 90 as depicted in FIG. 10is also generally indicative of the basic chopper modulated(un-balanced) pixel. The switching arrangement 220 includes the firstlogic circuit 222, the second logic circuit 224, the first switch 226, athird logic circuit 228, a fourth logic circuit 230, and a second switch232.

For normal operation of the thermopile 96, the Walsh function is tomodulate the electrical output of the pixel 90. In order for thethermopile 96 to provide the modulated electrical output therefrom, thememory cell 260 provides high output and the Walsh function is lowoutput. For example, the second logic circuit 224 generates low outputwhen it receives low output from first logic circuit 222 (e.g., whenhigh output is provided from memory cell 260) and when it receives lowoutput from the Walsh function that is set to zero. The first switch 226is off based on low output provided from the second logic circuit 224.

The fourth logic circuit 230 receives high output from the memory cell260 and high output from the third logic circuit 228. The high outputfrom the third logic circuit 228 is generated as a result of memory cell260 providing high output and Walsh function exhibiting low output. Theforth logic circuit 230 generates high output in response to receivinghigh output from memory cell 260 and third logic circuit 228 therebyactivating second switch 232 and allowing modulating electrical outputfrom the thermopile 96.

When the thermopile 96 is damaged, it is necessary for the first switch226 to be closed and the second switch 232 to be open. When first switch226 is closed and the second switch 232 is opened, modulated electricaloutput from previous pixel is routed through the first switch 226 andover the second switch 232 thereby bypassing the thermopile 96 and thesecond switch 232. To realize the above condition, memory cell 260provides low output. As such, the second switch 232 is always disabledbecause the fourth logic circuit 230 outputs low output. On the otherhand, the first switch 226 is always enabled (closed) since the firstlogic circuit 222 and the second logic circuit 224 always provides ahigh output if the memory cell 260 provides a low output.

FIG. 11 depicts a thermopile 96 and the switching arrangement 220 forthe voltage summing configuration of the array 76 in accordance to oneembodiment of the present invention. The pixel 90 as depicted in FIG. 11is also generally indicative of a balanced modulated pixel. Because thepixel 90 is a balanced modulated pixel, it is active all of the time.

The switching arrangement 220 includes the first logic circuit 222, thesecond logic circuit 224, the fourth logic circuit 230, the first switch226, the second switch 232, a third switch 234, and a fourth switch 236.For normal operation of the thermopile 96, the Walsh function is tomodulate the electrical output of the pixel 90. In order for thethermopile 96 to provide the modulated electrical output therefrom, thememory cell 260 provides a high output and the state of the Walshfunction can either be high output or low output. Such a conditionenables the thermopile 96 to provide a modulated electrical outputirrespective of the state of the Walsh function.

For example, the forth logic circuit 230 receives high output frommemory cell 260 and may receive high output from the Walsh function suchthat a low output is provided therefrom. The first switch 226 and thesecond switch 232 are open in response to low output from the fourthlogic circuit 230. The second logic circuit 224 receives high outputfrom the first logic circuit 222 and produces high output in responsethereto. The third switch 234 and the fourth switch 236 are closed inresponse to high output from the second logic circuit 224. When thethird switch 234 and the fourth switch 236 are closed, the thermopileproduces a reverse polarity modulated electrical output.

When the Walsh function provides low output, the fourth logic circuit230 provides high output and the second logic circuit 224 produces lowoutput. The first switch 226 and the second switch 232 are closed inresponse to high output from the fourth logic circuit 230 and the thirdswitch 234 and the fourth switch 236 are open in response to low outputfrom the second logic circuit 224. When the first switch 226 and thesecond switch 232 are closed, the thermopile produces a forward polaritymodulated electrical output.

When the thermopile 96 is damaged, which may produce an open circuit, itmay be necessary for the first switch 226, the second switch 232, thethird switch 234 and the fourth switch 236 to be closed to enable themodulated electrical output from a previous pixel to bypass thethermopile 96. To accomplish this, the memory cell 260 provides lowoutput causing the fourth logic circuit 230 to produce a high output(irrespective of state of Walsh function) and the second logic circuit224 to produce high output (irrespective of state of Walsh function). Ahigh output from the second logic circuit 224 and the fourth logiccircuit 230 causes the first switch 226, the second switch 232, thethird switch 234, and the fourth switch 236 to close thereby bypassingthe thermopile 96.

FIG. 12 depicts the thermopile 96 and the switching arrangement 220 forthe current summing configuration of the array 76′ in accordance to oneembodiment of the present invention. The pixel 90 as depicted in FIG. 12is generally indicative of a chopper modulated unbalanced pixel. Becausethe pixel 90 is unbalanced, it is active only half of the time.

The switching arrangement 220 includes the second logic circuit 224 andthe first switch 226. For normal operation of the thermopile 96, theWalsh function is to be modulated on the electrical output of the pixel90. In order for the thermopile 96 to provide the modulated electricaloutput therefrom, the memory cell 260 provides high output and the stateof the Walsh function is high output. As shown, the second logic circuit224 generates high output in response to the memory cell 260 providinghigh output and the Walsh function being a high output. The first switch226 closes in response thereto enabling the pixel 90 to produce themodulated electrical output.

When the Walsh function is low output, the second logic circuit 224produces a low output thereby opening the first switch 226 andpreventing the thermopile 96 from providing an electrical outputtherefrom. In contrast to the voltage summing configuration as noted inconnection with FIGS. 9-11, it is necessary to open the switch for aparticular pixel 90 that includes a damaged thermopile 96. Thiscondition ensures that pixels positioned in parallel with the damagedthermopile on a given row or column in the array 76′ is capable of stillproviding a modulated electrical output to the amplifier 102. To openthe first switch 226, the memory cell 260 provides low output when thethermopile 96 is detected to be damaged.

FIG. 13 depicts the thermopile 96 and the switching arrangement 220 forthe current summing configuration of the array 76′ in accordance to oneembodiment of the present invention. The pixel 90 as depicted in FIG. 13is also generally indicative of the chopper modulated unbalanced pixelthat is active half of the time.

The switching arrangement 220 includes the first switch 226 and thesecond switch 232. In order for the thermopile 96 to provide themodulated electrical output therefrom, the memory cell 260 provides highoutput and the state of the Walsh function is high output. As shown,when the Walsh function is high output and the memory cell 260 provideshigh output, the first switch 226 and the second switch 232 closethereby enabling the thermopile 96 to provide the modulated electricaloutput.

When the thermopile 96 is damaged, the memory cell 260 provides lowoutput thereby opening the second switch 232 and disabling thethermopile 96 to ensure that additional pixels on the same row or columnmay still continue to provide the modulated electrical output.

FIG. 14 depicts the thermopile 96 and the switching arrangement 220 forthe current summing configuration of the array 76′ in accordance to oneembodiment of the present invention. The pixel 90 as depicted in FIG. 14is also generally indicative of a balanced modulated pixel. Because thepixel 90 is a balanced modulated pixel, it is active all of the time.

The switching arrangement 220 includes the first logic circuit 222, thesecond logic circuit 224, the fourth logic circuit 230, the first switch226, the second switch 232, the third switch 234, and the fourth switch236. For normal operation of the thermopile 96, the Walsh function is tobe modulated on the electrical output of the pixel 90. In order for thethermopile 96 to provide the modulated electrical output therefrom, thememory cell 260 provides high output and the state of the Walsh functioncan either be high or low output.

For example, the forth logic circuit 230 receives high output frommemory cell 260 and may receive high output from the Walsh function suchthat high output is provided therefrom. The first switch 226 and thesecond switch 232 are closed in response to high output from the fourthlogic circuit 230. When the first switch 226 and the second switch 232are closed, the thermopile 96 produces a forward polarity modulatedelectrical output. The second logic circuit 224 receives low output fromthe first logic circuit 222 and produces low output in response thereto.The third switch 234 and the fourth switch 236 are open in response tolow output from the second logic circuit 224.

When the Walsh function provides low output, the fourth logic circuit230 provides low output and the second logic circuit 224 produces highoutput. The first switch 226 and the second switch 232 are open inresponse to the low output from the fourth logic circuit 230, and thethird switch 234 and the fourth switch 236 are closed in response tohigh output from the second logic circuit 224. When the third switch 234and the fourth switch 236 are closed, the thermopile 96 produces areverse polarity modulated electrical output.

When the thermopile 96 is damaged, it is necessary for the first switch226, the second switch 232, the third switch 234 and the fourth switch236 to be open such that the thermopile 96 is bypassed to enable themodulated electrical output from a previous pixel. To accomplish this,the memory cell 260 provides low output causing the fourth logic circuit230 to produce low output (irrespective of state of Walsh function) andthe second logic circuit 224 to produce low output (irrespective ofstate of Walsh function). A low output from the second logic circuit 224and the fourth logic circuit 230 causes the first switch 226, the secondswitch 232, the third switch 234, and the fourth switch 236 to openthereby disabling the thermopile 96 to ensure that additional pixels onthe same row or column may still continue to provide the modulatedelectrical output.

While embodiments of the invention have been illustrated and described,it is not intended that these embodiments illustrate and describe allpossible forms of the invention. Rather, the words used in thespecification are words of description rather than limitation, and it isunderstood that various changes may be made without departing from thespirit and scope of the invention.

What is claimed is:
 1. A detector comprising: at least one thermalsensing element being configured to: receive at least one oscillatingsignal; detect at least a portion of a thermal output from an object;generate an electrical output signal indicative of the detected thermaloutput; and modulate the electrical output signal with the at least oneoscillating signal to provide a modulated output signal indicative of atleast a portion of the image of the object; a converter configured toconvert the modulated output signal into a digital modulated outputsignal; and a demodulation circuit configured to demodulate the digitalmodulated output signal to provide at least a portion of an image of theobject.
 2. The detector of claim 1 wherein the at least one thermalsensing element includes: a thermopile configured to detect the at leasta portion of the thermal output from the object; at least one firstswitch configured to receive the at least one oscillating signal formodulating the electrical output signal with the at least oneoscillating signal; and at least one second switch configured to enablethe at least one thermal sensing element to provide an electrical outputtherefrom in the event the thermopile is damaged.
 3. The detector ofclaim 2 wherein the thermopile is formed with (Bi_(1-x)Sb_(x))₂(Te_(1-y)Se_(y))₃, wherein x and y are each one of an integer and anon-integer.
 4. The detector of claim 2 wherein the thermopile is formedof a superlattice quantum well structure.
 5. The detector of claim 1wherein the at least one thermal sensing element includes a firstthermal sensing element and a second thermal sensing element that arecoupled in series with one another.
 6. The detector of claim 5 furthercomprising an amplifier that is connected in series with the firstthermal sensing element and the second thermal sensing element forreceiving the modulated output signal from each of the first thermalsensing element and the second thermal sensing element.
 7. The detectorof claim 1 wherein the at least one oscillating signal comprises atleast one Walsh function.
 8. The detector of claim 1 wherein thedemodulation circuit comprises one of a Fast Walsh Transform, a FastFourier Transform and a matched filter.
 9. A detector comprising: atleast one thermal sensing element being configured to: receive at leastone oscillating signal; receive at least a portion of a thermal outputfrom an object; provide an electrical output signal indicative of thereceived thermal output; and modulate the electrical output signal withthe at least one oscillating signal to provide a modulated outputsignal; a converter configured to convert the modulated output signalinto a digital modulated output signal; and a demodulation circuitconfigured to demodulate the digital modulated output signal to provideat least a portion of an image of the object.
 10. The detector of claim9 wherein the at least one thermal sensing element includes: athermopile configured to detect the at least a portion of the thermaloutput from the object; at least one first switch configured to receivethe at least one oscillating signal for modulating the electrical outputsignal with the at least one oscillating signal; and at least one secondswitch configured to enable the at least one thermal sensing element toprovide an electrical output therefrom in the event the thermopile isdamaged.
 11. The detector of claim 10 wherein the thermopile is formedwith (Bi_(1-x)Sb_(x))₂ (Te_(1-y)Se_(y))₃, wherein x and y are each oneof an integer and a non-integer.
 12. The detector of claim 10 whereinthe thermopile is formed of a superlattice quantum well structure. 13.The detector of claim 9 wherein the at least one thermal sensing elementincludes a first thermal sensing element and a second thermal sensingelement that are coupled in series with one another.
 14. The detector ofclaim 13 further comprising an amplifier that is connected in serieswith the first thermal sensing element and the second thermal sensingelement for receiving the modulated output signal from each of the firstthermal sensing element and the second thermal sensing element.
 15. Thedetector of claim 9 wherein the at least one oscillating signalcomprises at least one Walsh function.
 16. The detector of claim 9wherein the demodulation circuit comprises one of a Fast WalshTransform, a Fast Fourier Transform and a matched filter.
 17. A detectorcomprising: at least one thermal sensing element including a thermopileand being configured to: receive at least one oscillating signal;receive at least a portion of a thermal output from an object; providean electrical output signal indicative of the received thermal output;and modulate the electrical output signal with the at least oneoscillating signal to provide a modulated output signal; a converterconfigured to convert the modulated output signal into a digitalmodulated output signal; and a demodulation circuit configured todemodulate the digital modulated output signal to provide at least aportion of an image of the object.
 18. The detector of claim 17 whereinthe thermopile is formed of a superlattice quantum well structure. 19.The detector of claim 17 wherein the at least one thermal sensingelement includes a first thermal sensing element and a second thermalsensing element that are coupled in series with one another.
 20. Thedetector of claim 19 further comprising an amplifier that is connectedin series with the first thermal sensing element and the second thermalsensing element for receiving the modulated output signal from each ofthe first thermal sensing element and the second thermal sensingelement.